Teach navigation throttles how to cancel requests in WillProcessResponse.

content/public/browser/navigation_throttle.h

Index: content/public/browser/navigation_throttle.h
diff --git a/content/public/browser/navigation_throttle.h b/content/public/browser/navigation_throttle.h
index fff64b1f4fb8c61f425c27b140786fce5eaa617d..5876343db0decd9bb6933f6243299aa81b825d06 100644
--- a/content/public/browser/navigation_throttle.h
+++ b/content/public/browser/navigation_throttle.h
@@ -55,6 +55,15 @@ class CONTENT_EXPORT NavigationThrottle {
   // CANCEL_AND_IGNORE or DEFER and perform the destruction asynchronously.
   virtual ThrottleCheckResult WillRedirectRequest();
 
+  // Called when a response's headers and metadata are available.
+  //
+  // The implementer is responsible for ensuring that the WebContents this
+  // throttle is associated with remain alive during the duration of this
+  // method. Failing to do so will result in use-after-free bugs. Should the
+  // implementer need to destroy the WebContents, it should return CANCEL,
+  // CANCEL_AND_IGNORE and perform the destruction asynchronously.
+  virtual ThrottleCheckResult WillProcessResponse();
+
   // The NavigationHandle that is tracking the information related to this
   // navigation.
   NavigationHandle* navigation_handle() const { return navigation_handle_; }